This invention relates generally to a ballast for a fluorescent lamp, and more particularly to an anti-flicker scheme for a fluorescent lamp ballast driver.
A fluorescent lamp is powered by a ballast. The ballast can be of the magnetic or electronic type. Electronic ballasts include a driver for controlling the operation of the ballast. In order to lower costs and improve reliability, more and more of the components within the driver are included within an integrated circuit. The voltage source for the integrated circuit is derived from the A.C. mains and supplied to a VDD pin of the integrated circuit. A ballast which includes such an integrated circuit is made by Philips Electronics North America Corporation under its ECOTRON trademark.
Lamp flicker can be caused by the integrated circuit turning off momentarily due to the voltage level at the VDD pin falling below a minimum threshold required to power the integrated circuit. The voltage at the VDD pin generally decreases and can fall below the minimum threshold after preheating the lamp electrodes during lamp turn on (i.e. during lamp ignition). More particularly, the ballast draws more current during lamp turn on which can cause the voltage supplied by the mains to the ballast to momentarily dip. The momentary reduction of the mains voltage can result in the voltage level at the VDD pin falling below the minimum threshold to power the integrated circuit and the consequential lamp flicker.
Flicker can be a particular problem when the electronic ballast is used in combination with a triac dimmer. The triac dimmer at large cut-in angles, that is, at low dim settings, often can result in a VDD pin voltage near the minimum threshold for powering the integrated circuit. The high cut-in angles often permit development of a sufficient VDD pin voltage to preheat the lamp electrodes (filaments) but do not permit development of a sufficient VDD pin voltage to ignite the lamp. Consequently, cut-in angles must be reduced (i.e. light level settings must be increased) to increase the VDD pin voltage so as to avoid flicker. Restriction in the minimum triac dim setting results.
Accordingly, it is desirable to provide an improved fluorescent lamp ballast driver which avoids lamp flicker due to the momentary dip in mains voltage during lamp turn on. The improved fluorescent lamp ballast driver should include an anti-flicker scheme which permits operation of the lamp at low triac dim settings. The anti-flicker scheme should particularly address the different lamp operating conditions during and after preheat of the lamp electrodes.